The present invention relates to a power control arrangement for controlling the power output of a plurality of resistive heating elements.
It is known to control the power output of a resistive heating element in a number of ways. It is known from U.S. Pat. No. 4,334,147 to use burst firing techniques in which electronic switching is employed to control the power applied to the heating element by controlling the number of conductive half-cycles during which power is applied to the heating element each control period. However, burst firing techniques encounter problems with flicker when the heating element has a fast thermal response time and a highly variable resistance with temperature. In order to reduce flicker to an acceptable level the control period should be as short as possible, say five half-cycles, but as the control period becomes shorter the minimum power level rises and at one half-cycle in five the power level is 29 percent of full power. For an electric cooker having a heating element of, say, 1000 watts a minimum power output of 290 watts does not provide sufficient control.
It is also known from U.K. Patent Application No. 2 132 060 A to connect four infra-red lamps in both series and parallel, together with a diode at some power settings, to provide a minimum power level of seven percent of full power. Such an arrangement works adequately when four infrar-red lamps are provided and a satisfactory range of power output can be attained. However, infra-red lamps are expensive and it is therefore desirable to reduce the number of lamps employed, but reducing the number of lamps in this prior publication brings about a severe restriction on the range of power output.